1. Field of the Invention
The present invention relates to an inter-vehicle distance detecting device and an inter-vehicle distance detecting method, and more particularly relates to a device which is preferably used to detect an inter-vehicle distance from a user's own vehicle to a neighboring vehicle by means of an onboard camera and an onboard radar.
2. Description of the Related Art
There recently have been provided systems which detect neighboring vehicles by means of onboard cameras and radars, show positional information on the detected neighboring vehicles to a user, and/or control steering according to the status of such information. There have also been provided systems which detect neighboring vehicles by means of cameras and radars, and conduct vehicle following by controlling a brake and an accelerator according to a result thereof to keep a constant inter-vehicle distance to a preceding vehicle. Any of these systems necessarily detect the distance from the user's own vehicle to a neighboring vehicle.
According to the method using a radar, the distance from the user's own vehicle to the neighboring vehicle is measured based on a time of travel of a radio wave such as a millimeter radio wave from emission to reception after reflection by the neighboring vehicle. According to the method using cameras, stereo cameras including a pair of cameras spaced apart are used, for example. According to this method, a pair of images of a neighboring vehicle acquired by the pair of cameras are compared to detect a difference between the images, thereby measuring the distance to the neighboring vehicle according to the principle of triangulation.
According to the method which uses radar, it is possible to accurately measure the distance to a neighboring vehicle. However, a neighboring vehicle might not be detected due to the influence of multipath where a direct wave, which is reflected by the neighboring vehicle and directly returns to the radar, and an indirect wave, which is further reflected by a road surface or the like and indirectly returns to the radar, cancel out each other, resulting in the reduction of a received signal strength. On the other hand, according to the method which uses cameras, though the precision of the measured distance is inferior to that measured by the radar, the above-described multipath phenomenon where a neighboring vehicle cannot be detected will not occur.
As a possible method to avoid the state where the distance to a neighboring vehicle cannot be measured, if a neighboring vehicle can be detected by the radar, the distance to the neighboring vehicle is measured based on the information from the radar, and if the radar becomes unable to detect the neighboring vehicle, the distance is measured by images acquired by the cameras. However, there arises a problem that the precision of the measured distance decreases if the radar cannot detect the neighboring vehicle and the radar is simply switched over to the cameras.
There is proposed a technology which calculates respective reliabilities for an inter-vehicle distance measured based on data acquired by a radar, and an inter-vehicle distance measured based on data acquired by cameras, and employs a detected inter-vehicle distance with a higher reliability (Japanese Unexamined Patent Publication No. H6-230115, for example). However, the technology disclosed by Japanese Unexamined Patent Publication No. H6-230115 only calculates the reliability of the distance measured by the cameras, and cannot increase the precision of the measured distance.